The present invention relates to an improvement in a method for making a photoresist stripping solution, and, more particularly, to a method for making a photoresist solution comprising a blend of at least one organic sulfonic acid wherein concentrations of trace amounts of residual sulfuric acid and sulfur trioxide in the blend are reduced to very low levels.
During manufacture of semiconductors and semiconductor microcircuits, it is frequently necessary to coat the materials from which the semiconductors and microcircuits are manufactured with a polymeric organic substance, generally referred to as a photoresist, e.g., a substance which forms an etch resist upon exposure to light. These photoresists are used to protect selected areas of the surface of the substrate, e.g. silicon, SiO2 or aluminum while such etchant selectively removes the substrate materials from the unprotected area of the substrate. Following completion of the etching operation and washing away of the residual etchant, it is necessary that the resist be removed from the protective surface to permit essential finishing operations. It is necessary in a photolithographic process that the photoresist material, following pattern delineation, be evenly and completely removed from all unexposed areas, in the case of positive resists, or exposed areas in the case of negative resists, so as to permit further lithographic operations. Even the partial remains of a resist in an area to be further patterned are undesirable. Also, undesired resist residues between patterned lines can have deleterious effects on subsequent processes, such as metallization, or cause undesirable surface states and charges.
A common method used in removing the photoresist from the substrate is by contacting the substrates with an organic stripper. Heretofore these organic strippers have been composed of various components whose purpose it was to lift and remove the polymeric photoresist from the substrate. However, these stripping solutions have heretofore usually contained chlorinated hydrocarbon compounds which resulted in a distinct disadvantage due to the toxicity as well as pollution problems arising from their disposal.
It is also highly desirable that stripping compositions be provided that are effective and efficient for removal of photoresist coatings from the substrate without attacking the underlying metal surfaces.
It is also desirable that effective stripping compositions be provided that are devoid of undesirable chlorinated solvents considered to be undesirable by regulatory agencies overseeing their production and use.
It is known that mixtures of aromatic solvents with an alkylarylsulfonic acids having 6-20 carbons effectively removing positive and negative tone photoresists, bonding adhesive, ink mark and post etch residue etc., are being used in semiconductor manufacturing. However, such mixtures can cause corrosion of metal surface substrates, such as aluminum and copper, which are being used as conductive wiring for the integrated devices. Compositions have been developed by adding corrosion inhibitors to reduce the effect of metal corrosion, such as described in the following references:                U.S. Pat. No. 4,215,005—discloses the use of hydrogen fluoride complexes to reduce aluminum corrosion during stripping to improve over stripping solutions cited in U.S. Pat. No. 4,165,294.        U.S. Pat. No. 4,221,674—discloses the use of inhibitors system containing hydrogen fluoride and nitrile compound to enhance corrosion inhibition properties of the fluoride to further improve over stripping solutions cited in U.S. Pat. No. 4,215,005.        U.S. Pat. No. 4,395,348—discloses the use of catechol as corrosion inhibitor by heating the solution to 60° C. to dissolve the catechol into the solution prior using it for photoresist stripping.        U.S. Pat. No. 4,491,530—discloses the use of water soluble sulfone to suppress the “brown staining” which is a form of metal corrosion on aluminum metal surface [Col 5, lines 19-30].        U.S. Pat. No. 4,844,832—discloses the use of phenolic compounds in the stripping solution to achieve the stripping performance.        U.S. Pat. No. 5,728,664—discloses the use of alkyl phenol as corrosion inhibitor to improve stripper solutions cited in U.S. Pat. No. 4,992,108.        U.S. Pat. Nos. 6,475,292 and 6,660,460—discloses use of catechol as the preferred corrosion inhibitor.        
Kirk-Othmer Encyclopedia of Chemical Technology: Wiley InterScience, Vol 23, describes details of the sulfonation process for aromatic hydrocarbon. Aromatic hydrocarbons are generally directly sulfonated using sulfur trioxide, oleum, or sulfuric acid. The main cause of the attack comes from residual amounts of sulfuric acid and sulfur trioxide that are present in the sulfonic acid from the manufacturing.SO3+H2OH2SO4 Al+H2SO4Al2(SO4)3 
By mixing the sulfonic acid with a hydrocarbon solvent at an elevated temperature for an extended period of time, residual amounts of sulfuric acid and sulfur trioxide react with the hydrocarbon solvent to form a corresponding organic sulfonic acids and the amounts of each undesirable component are thereby reduced to an acceptable and very low level that effectively eliminates the attack on aluminum.

It is an object of the invention to provide an improved method of manufacturing of photoresist stripping compositions comprising one or more organic sulfonic acids and one or more hydrocarbon solvents which can be used at a wide range of operating conditions and at lower temperatures without corrosion of the semiconductor substrate. The improvement comprises blending the one or more organic sulfonic acids with the one or more hydrocarbon solvents at elevated temperature for a period of time sufficient for the residual amounts of sulfuric acid and sulfur trioxide to react with the one or more hydrocarbon solvents to form corresponding organic sulfonic acids.
It is another object of this invention to provide a photoresist stripping solution comprising an organic sulfonic acid and a hydrocarbon solvent which is essentially free of chlorinated hydrocarbon compounds and which can be used at a wide range of operating conditions and at lower temperatures without corrosion of the semiconductor substrate.
The material to be stripped is typically contacted with the present compositions for a period of time sufficient to at least partially remove the material, preferably for a period of time to substantially remove the material, and more preferably for a period of time to completely remove the material. The contact time of the present compositions with the material to be stripped will vary, depending upon the exact stripper composition as well as the material to be stripped.
For example, the material to be removed may be contacted with the present compositions for up to about 60 minutes, preferably from about 5 seconds to about 45 minutes, and more preferably from about 15 seconds to about 35 minutes. In removing material, the compositions of the present invention may be used at room temperature or may be heated. Such heating has the advantage of shortening the time required for complete removal of the material. Typically, the compositions of the present invention are heated at a temperature of about 30° C. to about 120° C., and preferably about 40° C. to about 95° C. It is preferred that the compositions of the present invention are heated.